A punctal plug is a small medical prosthesis that is inserted into the lacrimal (tear) drainage duct (punctum) of an eye to block the duct. Blocking the duct prevents the drainage of liquid from the eye into the duct. They are used for dry eye or to deliver therapeutic agent. Other kinds of prostheses are also known in the medical arts for placement in natural lumens. For example urethral implants for delivery of drugs.